Organic EL display devices are known as thin profile, high image quality, and low power consumption display devices. The organic EL display device has formed therein a plurality of pixel circuits arranged in a matrix, the pixel circuits including organic EL elements, which are light-emitting electrooptical elements driven by current, driving transistors, and the like.
The organic EL elements have been known for having a decrease in light emitting efficiency due to deterioration over time, resulting in a decrease in light-emitting luminance. FIG. 18 is a drawing for describing the effect that deterioration over time of the organic EL elements has on image display. More specifically, FIG. 18(A) shows a situation in which the same pattern is displayed over a long period of time, and FIG. 18(B) shows a situation in which all pixel circuits are applied a signal for the same luminance after the same pattern was displayed over the long period of time. As shown in FIG. 18(A), the cumulative light-emitting time for organic EL elements (hereinafter, “organic EL elements in a first region PA”) in the pixel circuits in the region PA (hereinafter, the “first region”) where bright display is performed over a long period time is longer than that of organic EL elements (hereinafter, “organic EL elements in a second region PB”) in pixel circuits in the region PB (hereinafter, the “second region”) where dark display is performed over a long period of time. Thus, the organic EL elements in the first region PA undergo a decrease in light-emitting efficiency due to greater deterioration than those in the second region PB. As a result, as shown in FIG. 18(B), so-called screen burn-in occurs in the first region PA. Specifically, display of the same luminance as the second region PB normally should occur in the first region PA, but display of a lower luminance than the second region PB occurs in the first region PA.
FIG. 19 is a drawing for describing the decrease in luminance of the organic EL elements. Here, a fixed current is assumed to be fed to the organic EL elements. As deterioration over time of the organic EL elements progresses, impedance increases in the organic EL elements. As a result, as shown in FIG. 19, forward-biased voltage applied to the organic EL elements increases as deterioration over time of the organic EL elements progresses. As described above, light-emitting efficiency decreases as deterioration over time of the organic EL elements progresses, and as a result, the decrease in luminance occurs as shown in FIG. 19. The deterioration over time of the organic EL elements in the second region PB has not progressed as much as those in the first region PA, and thus, there is not as much decrease in luminance in the second region PB. On the other hand, the deterioration over time of the organic EL elements in the first region PA has progressed more than in the second region PB, and thus, there is a greater decrease in luminance in the first region PA. As a result, the display state shown in FIG. 18(B) occurs.
In relation to the present invention, Patent Document 1 discloses a pixel circuit that compensates for increase in forward bias voltage resulting from deterioration over time of organic EL elements. FIG. 20 is a circuit diagram showing a configuration of the pixel circuit 91 disclosed in Patent Document 1. In FIG. 20, for ease of explanation, the reference characters in the drawing of Patent Document 1 are modified. A pixel circuit 91 has one organic EL element OLED, six transistors T11 to T16, two capacitors C11 and C12, and a variable bias voltage source VS. The transistor T12 is of a p-channel type, and the transistors T11 and T13 to T16 are of an n-channel type.
First, a scan wiring line Sj is selected and the transistor T11 turns ON, and a voltage based on the data signal fed from a data wiring line Di is written to the capacitor C11. Next, the selection of the scan wiring line Sj ends and the transistor T11 turns OFF, and control lines Vg13j and Vg15j are selected. As a result, the transistor T13 turns ON, and a drive current based on a voltage between source and gate of the transistor T12 is fed to the organic EL element OLED. Also, the transistor T15 turns ON, and the gate potential of the transistor T16 becomes equal to the anode potential of the organic EL element OLED based on the drive current. The anode potential Pi of the organic EL element OLED changes due to deterioration of the organic EL element OLED. Here, using the variable bias voltage source VS, a source potential Ps of the transistor T16 is set according to the following formula (1).Ps=Pi−Vth  (1)
Here, Vth represents a threshold voltage of the transistor T16.
By setting the source potential Ps of the transistor T16 according to formula (1), it is possible to extract the increase in forward bias voltage resulting from the deterioration of the organic EL element OLED as the source/drain current of the transistor T16. After the source/drain voltage of the transistor T16 is determined, the selection of the control line Vg15j ends and the transistor T15 turns OFF, and then the control line Vg14j is selected and the transistor T14 turns ON. Thus, the potential of the gate terminal of the transistor T12 decreases based on the source/drain current of the transistor T16. As a result, it is possible to perform luminance compensation based on the increase in forward bias voltage resulting from deterioration over time of the organic EL element OLED. Therefore, it is possible to mitigate a decrease in luminance of emitted light resulting from the deterioration over time of the organic EL elements OLED.